Sensors which operate on the basis of optical phenomena have found uses in various fields. Examples include speed, proximity and electrical current sensors which use magneto-optic crystals, strain gauge sensors which use birefringent crystals, and temperature sensors which use fluorescent crystals. This diverse class of sensors typically has in common the use of some type of optical fiber for the purpose of transmitting a light beam from a suitable light source to an optical sensing element, and from the optical sensing element to a suitable photodetector, with which an optical signal can be detected to evaluate the particular condition being sensed. Generally, optical fibers operate on the basis of total internal reflection resulting from the different refractive indices of the fiber core and the cladding material which surrounds the fiber core.
For many applications, an optical sensor is constructed such that the individual components are axially aligned, as schematically represented by FIG. 1. FIG. 1 illustrates a magnetic field sensor 10 which operates by sensing the modulation of a magnetic field created by a magnet 26. Such an arrangement is useful to sense rotation by mounting the magnet 26 on a rotating shaft (not shown). As the shaft rotates, the magnetic field in the vicinity of the sensor 10 increases and decreases as the magnet 26 approaches and retreats from, respectively, a Faraday material which serves as a sensing element 18 in the sensor 10.
Faraday materials are well known in the art to exhibit what is termed Faraday rotation. Faraday rotation is the change in the polarization of an electromagnetic wave as it travels through certain substances. With respect to sensing magnetic fields with optical sensors, the phenomenon is often referred to as magneto-optical rotation, which is the tendency of a magnetic field to rotate the plane of polarization of light passing through a substance. Therefore, as polarized light passes through a Faraday material, the linear polarization vector of the light will rotate in response to a nearby magnetic field. The degree of rotation is generally proportional to the strength of the magnetic field.
Conventionally, the optical sensor 10 requires a suitable light source 12, such as an LED or laser, an input optical fiber 28 to deliver the light beam to the Faraday material 18, and a return optical fiber 30 which delivers the altered optical signal to a suitable photodetector device 24. In addition, the optical sensor 10 typically includes a grated index (GRIN) lens 14 which serves to collimate the optical signal from the light source 12, and a first linear polarizer 16 which serves to confine the transverse waves of the optical signal to a specific plane prior to entering the sensing element 18. The optical sensor 10 also includes a second linear polarizer 20 and a second GRIN lens 22 for the optical signal as it leaves the Faraday material and travels toward the photodetector device 24. The second polarizer 20 generally serves as an analyzer to determine the degree of rotation of the light beam after it has passed through the sensing element 18. With respect to the first polarizer 16, the second polarizer 20 is angularly offset such that changes in the optical signal corresponding to the modulation of the magnetic field will be more readily detected by the photodetector 24.
As one would expect, the streamlined arrangement of the optical sensor 10 is not well suited for applications that have minimal space to accommodate a sensor, such as a speed sensor used in an automotive anti-lock braking system. Consequently, optical sensors 32 such as the one shown in FIG. 2 have been developed. This optical sensor 32 is constructed to receive both optical fibers 28 and 30 on one side of a sensor housing 34 so as to facilitate the use of the sensor 32 within smaller installations. Also notable with this design is the use of a single magnet 26 mounted stationary with respect to the sensor 32, wherein modulation of the magnetic field is induced by a toothed wheel (not shown) which rotates within the magnetic field.
Again, the sensor 32 includes a pair of GRIN lens 14 and 22, as well as a pair of linear polarizers 16 and 20. In addition, such sensors 32 are known to include prisms 15 and 21. The inclined faces 15a and 21a of the prisms 15 and 21 serve to reflect the light beam at right angles so as to enable the light beam to negotiate the 180 degree turn within the housing 34.
While the above arrangement may be suitable for some applications, the added complexity of the construction as well as the precision required to optically align the individual components of the sensor 32 are undesirable for mass production. Furthermore, high component costs are associated with the sensor 32 of FIG. 2, as well as significant levels of optical losses.
Alternatively, it is known in the art to extend the optical fibers 28 and 30 further into the housing such that the optical fibers 28 and 30 serve as optical waveguides in place of the prisms 15 and 21. To do so, the input fiber 28 must be bent sufficiently such that its fiber end is aligned with the Faraday material to redirect the light beam into the sensing element 18. Similarly, the return optical fiber 30 must be bent sufficiently such that its fiber end is aligned with the opposite side of the sensing element 18, thereby permitting the light to be received from the sensing element 18 and redirected so as to travel in a direction substantially parallel to the direction in which the light beam entered the housing.
While a benefit to this approach is the elimination of the prisms 15 and 21, a significant disadvantage is that, at a sharp bend within the optical fibers 28 and 30, much of the light is absorbed by the cladding material which surrounds the optical fiber core. This loss occurs because the light travelling through the core will be incident on the cladding material at an angle which exceeds the maximum angle for total internal reflection. As a result, the use of an optical fiber as a waveguide under these circumstances produces a sensor which is lossy (i.e., characterized by high attenuation per unit length), causing the optical signal produced by the sensing element 18 to be significantly attenuated, such that the sensitivity of the sensor is reduced. Furthermore, producing sharp bends in an optical fiber without damaging or misaligning the optical fiber necessitates skilled labor to assemble the sensor, which frustrates the ability to mass produce such a sensor.
As a further disadvantage, while large core optical fibers are desirable because they have the potential for being lower cost, reducing connector losses and costs, and are able to launch more power into the sensing element, sharp bends in large core optical fibers (e.g., cores as large as about 1 millimeter) are susceptible to breakage as stress is relieved over time. Accordingly, large core optical fibers cannot be readily used as a waveguide in applications that require a small size optical sensor.
Thus, it would be desirable to provide a low cost, high volume optical sensor which is constructed so as to enable the sensor to be suitable for applications whose size requirements are limited, while requiring a minimal number of optical components, and without requiring the use of an optical fiber as the waveguide to and from the sensing element.